The art of ink-jet printing is nowadays relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink-jet technology for producing printed media.
Generally an ink-jet image is formed when a precise pattern of dots is ejected from a drop-generating device known as a “printhead” onto a printing medium, typically a paper sheet. Typically, an ink-jet printhead is supported on a movable carriage that traverses over the surface of the paper sheet and is controlled to eject drops of ink at appropriate times pursuant to commands of a microprocessor or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed.
The ink jet print head of an ink jet printer generally comprises a substrate, a layer defining ink passage ways, usually named in the art as “barrier layer” and a nozzle plate. The substrate is generally made of silicon. A plurality of thin film layers is deposited on a face of the silicon substrate to make up the active electronic components, the ejection actuators, the conductive traces, and the protective elements. The ejection actuators are substantially of two kinds, thermal actuators and mechanical actuators. The thermal actuators provide the energy to eject the ink drop by means of the heat provided by a resistor which vaporize the ink contacting the resistor surface. The mechanical actuators provide the energy to eject the ink drop by means of the vibration of a lamina which mechanically ejects the ink. The substrate more particularly includes a top layer of tantalum having a protective and anti-cavitation action.
The barrier layer is generally made of a photopolymer. Using photolithographic techniques, the ejection chambers and the microidraulic channels which represent the passage ways for the ink delivery and storage are realized in the photopolymer barrier layer. The nozzle plate is generally made of a plastic material, such as, for example, polyimide, or a metallic material, such as, for example, palladium plated nickel, rhodium plated nickel, or gold plated nickel. The nozzle plate provided with ejection nozzles made in correspondence with the ejection resistors and the ejection chambers is attached to the barrier layer.
In recent years, the nozzle plate has been made integrally with the barrier layer. When the layer defining ink passage ways includes both the barrier layer and the nozzle plate, such a layer is known in the art as a “structural layer”. In such a case, the manufacturing process includes a step of forming a pattern of the ejection chambers and the microidraulic channels with a soluble resin or a metal, a step of coating a photopolymer covering the soluble resin or metal pattern, a step of forming orifices in the photopolymer in correspondence of the ejection chambers over the ejection resistors, a step of curing the photopolymer, and a step of dissolving the soluble resin or metal.
A main concern related to the foregoing ink-jet printhead architecture includes delamination of the polymeric layer defining ink passage ways (i.e., the barrier or structural layer) from the substrate and/or from the nozzle plate. Delamination principally occurs due to the action of environmental moisture and ink which are in continuous contact with the edges of the interface between the polymeric layer and the substrate or the nozzle plate in the drop generator regions.
The adhesive characteristics of tantalum are due to the fact that such a metal is easily oxidized by the oxygen contained in the atmosphere. The tantalum oxide is able to form chemical bonds with the polymeric material of the barrier or structural layer. However, the chemical bond between tantalum oxide and a polymer film tends to be easily degraded by water, since the water forms a hydrogen bond with the oxide that competes with and replaces the original polymer to oxide bond, and thus ink formulations debond an interface between tantalum oxide and a polymer barrier.
In particular, a solvent, such as water, from the ink enters within the interface between the thin film substrate and the barrier layer and/or the interface between barrier layer and the nozzle plate, causing debonding of the interfaces through a chemical mechanism, such as hydrolysis, or a physical mechanism, such as swelling.
Moreover, new developments in ink chemistry have resulted in formulations containing additional components that more aggressively debond the interface between the thin film substrate and the barrier layer, as well as the interface between the barrier layer and the nozzle plate.
U.S. Pat. No. 6,659,596 and U.S. Pat. No. 7,048,359 disclose an ink jet printhead having a substrate comprising a plurality of thin film layers; a plurality of ink firing heater resistors defined in said plurality of thin film layers; a polymer barrier layer; and a carbon rich layer disposed on said plurality of thin film layers, for bonding said polymer barrier layer to said substrate. Both references disclose an improvement of the adhesion of the barrier layer to a tantalum layer.
Plasma processing is widely known processing technology that aims at modifying the chemical and physical properties of a surface by using a plasma-based material. Plasma processing includes plasma activation, plasma modification, plasma functionalization and plasma polymerization. Plasma processing is widely used in the field of electronics, automotive, textile, medical and aeronautic. A general review about plasma technology can be found on the Europlasma Technical Paper, “Functionalization of Polymer Surfaces”, dated May 8, 2004 and “Plasmapolymerisation. Pretreatment and finishing of polymer surfaces in the field of medical plastics” dated 20/09/04. Both articles have been downloaded on Oct. 13, 2006 from the Europlasma Internet site at http://www.europlasma.be/pageview.aspx?id=181&mid=17.